Limitations of MRSF-TDDFT for Applications in Photochemistry

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Photochemistry

Background:

• Mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) is a computational method for electronic structure.
• It aims to combine the efficiency of single-reference methods with the versatility of multireference methods for photochemical studies.

Purpose of the Study:

• To critically assess the general applicability of MRSF-TDDFT in photochemistry.
• To identify and analyze key limitations of the MRSF-TDDFT method for studying photochemical processes.

Main Methods:

• Critical assessment of the MRSF-TDDFT methodology.
• Analysis of the treatment of excited configurations within MRSF-TDDFT.
• Investigation of the behavior of triplet reference states in MRSF-TDDFT.

Main Results:

• MRSF-TDDFT omits some singly excited configurations while including doubly excited ones.
• The method yields unreliable excited-state energies when the triplet reference state undergoes abrupt character changes, such as near degeneracy of T1 and T2 states.
• These changes can cause discontinuities in potential energy curves for response states.

Conclusions:

• MRSF-TDDFT has significant limitations for general photochemistry applications.
• Strategies and diagnostics are proposed to detect these limitations during potential energy surface exploration and nonadiabatic molecular dynamics simulations.